The immune system has multiple pathways for recognizing and responding to microbial components and other disease-related stimuli. The dendritic-cell lineage of white blood cells controls this intricate system (Fig. 1). After nearly 35 years of research, there are many perspectives from which to appreciate the powerful influence of dendritic cells within the broad reach of immunology. From the perspective of natural selection, dendritic cells help the immune system defend against more than a thousand different forms of infection. They capture microbial proteins and lipids, and present them to lymphocytes, thereby launching lymphocyte responses. From a physiological perspective, resistance to infection is not a single automatic response. Instead, dendritic cells select from a host of rapid, short-lived innate reactions and from the more slowly acquired and longer-lived adaptive responses (Box 1).

From a cellular perspective, dendritic cells are best known for their role in initiating T-cell immunity, a role that I will emphasize in this Commentary. Nevertheless, dendritic cells can influence all types of lymphocytes, and, I suspect, they will eventually be found to affect some non-immune cells as well. From a medical perspective, dendritic cells influence many clinical conditions. In addition to providing resistance to some diseases, dendritic cells can instigate autoimmune inflammation and allergy and transplant rejection, and they can be exploited by several infections and tumors.